1. Field of the Invention
The present invention relates to an active type liquid crystal matrix display panel incorporating switching elements such as thin film transistors (hereinafter referred to as TFT's) and more specifically to the structure of a liquid crystal display TFT array substrate in which bus-bars interconnect the electrodes of the switching elements without breakage or deficiency when a large area TFT is formed by group exposure on a cell substrate of the display panel.
2. Description of the Prior Art
The structure and manufacturing process of the conventional typical TFT-loaded cell substrate for a display panel are described below with reference to FIGS. 1 and 2. FIG. 1 is a sectional view of an essential part of the conventional TFT structure and FIG. 2 is a plan view of the TFT structure of FIG. 1. A gate electrode 2, a gate insulating film 3 and a semiconductor layer 4 are sequentially deposited on an insulating substrate 1 which also serves as a display cell substrate, and a source electrode 5 and a drain electrode 6 are formed on the semiconductor layer 4, so as to manufacture a TFT. The drain electrode 6 is connected to a display picture element electrode 7.
The TFT's with the above structure are normally arranged in a matrix on the cell substrate to produce a liquid crystal display cell. The TFT's are connected to a driving circuit to serve as switching elements of a liquid crystal X-Y matrix display panel, controlling power supply to each picture element electrode 7. With the above construction, in order to realize a liquid crystal display panel with good matrix display characteristic and uniform contrast for all picture elements, it is required to manufacture a TFT array substrate in which the TFT's have good and uniform characteristics. Meanwhile, in order to obtain a picture having high resolution and a bright pattern over a large display area, it is necessary to form multiple small TFT's over the entire display area. For this purpose, photolithography for patterning a large area thin film with a good accuracy is normally required demanded. However, as a thin film area becomes larger, the accuracy in resist patterning using a photo mask and a mask aligner is poorer and accurate patterning of the thin film is more difficult. As the accuracy of the pattern thin film deteriorates the amount of overlay between the gate electrodes and source/drain electrodes fluctuate, resulting in variable capacities between the gate electrodes and source/drain electrodes. Large position deviation cause offset between the gate electrodes and the source/drain electrodes, deteriorating TFT characteristics. For highly accurate patterning of TFT-loaded large area display region, a group exposure method has been proposed in which the TFT's in the display region are divided into a plurality of groups and patterns are formed sequentially by group. In this group exposure method, however, each pattern can deviate with respect to the others at the group boundaries, causing mismatching between bus-bars in connecting the gate electrodes in the X-direction and the source electrodes in the Y-direction of the patterns. This results in partially narrow or broken bus-bars.
The bus-bars for transmitting signals to the gate and source electrodes in the each TFT element are aligned in X- and Y-directions along the gaps between the TFT's in a matrix. Each bus-bar must be a straight strip for interconnecting the gate electrodes on the same line or the source electrodes in the same row at a time. When the gate electrode bus-bars and source electrode bus-bars are formed by patterns using a group exposure method with positive resists, mismatching between the bus-bars tends to occur at the group exposure boundaries as shown in FIG. 3. Then, the bus-bars become narrower at the boundaries. If deviation between the bus-bars is excessive, connection between the bus-bars is broken. In fact, the boundaries of two adjacent exposure groups overlap each other, so that each group boundary is subjected to exposure two times. In actual group exposure, therefore, the bus-bars deviate from each other leaving a narrowed connecting portion 40 shown in FIG. 4.